1. Field of the Invention
The present invention relates to a flame sensor. More particularly, the present invention relates to a flame sensor capable of detecting a flame in place where solar rays or artificial rays of light such as halogen lamp are present without being affected by such rays of light.
2. Description of the Related Art
To detect a flame, there is a convenient method that detects resonance radiation generated by a high-temperature carbonic acid gas contained in the flame, as is well known in the art. A line spectrum of resonance radiation of the carbonic acid gas includes many wavelengths. To discriminate the line spectrum from ordinary artificial illumination and solar rays, it is appropriate to utilize a spectral line within the range of the infrared region or the ultraviolet region for detecting the flame. Because optical components belonging to the infrared region or the ultraviolet region do not much exist in artificial rays of light such as illumination, so disturbance by external light when sensing a flame is less in these regions.
To detect a flame in the presence of solar rays, a conventional method detects the line spectrum due to resonance radiation of the carbonic acid gas generated by the flame. To discriminate a continuous spectrum, such as solar rays and artificial light, from the line spectrum of the flame, this method compares and computes a plurality of outputs obtained from a monochromatic filter having a narrow-band that permits the passage of only the line spectrum of the flame and from monochromatic filters of a plurality of narrow-bands, which permit the passage of rays of light having one or a plurality of wavelengths, and the method discriminates whether light is the line spectrum of the flame or the continuous spectrum of the solar rays.
Another method utilizes flicker of light generated by the flame and detects the occurrence of the flame.
Among conventional methods that utilize resonance radiation of the carbonic acid gas, the method using the filter requires at least three monochromatic filters to achieve a flame sensor providing a small number of erroneous detections and capable of reliably sensing a flame. In addition, a computation circuit for sensing is complicated, and the flame sensor is unavoidably expensive.
Flame sensors using two or less filters involve the problem that the number of erroneous detections is great. Though economical, flame sensors utilizing the flicker of the flame also involve the problem that the number of erroneous detections is great.
Solar rays, artificial rays or radiation from a stove emit not only visible rays, but also radiation in the infrared regions. However, this radiation is a continuous spectrum. In contrast, the spectrum of resonance radiation of the carbonic acid gas generated by the flame is a line spectrum in which energy concentrates in extremely narrow regions.
Therefore, a flame sensor, using two filters, capable of reliably detecting a flame with equivalent certainty to the conventional flame sensors using three filters, is proposed (JP-A10-326391, JP-A2000-321132). This flame sensor utilizes the difference between the continuous spectrum and line spectrum for detecting the flame.
In this flame sensor, a broad band filter for permitting the passage of light of a band broader than a spectral line of resonance radiation of the carbonic acid gas generated by the flame and a narrow band filter for permitting the passage of only the spectral line of resonance radiation of the carbonic acid gas, are used. Intensity (optical energy) of light from the flame passing through these two filters is divided by the bandwidth of each filter to determine mean intensities.
When the intensity of the spectrum of light passing through the filters is a straight line-like continuous spectrum, energy of the rays of light passing through the two filters is proportional to the transmission bandwidth. Therefore, the mean intensities obtained by dividing this energy by the bandwidth are equal for the two filters.
However, when the rays of light passing through the filters are the line spectrum of resonance radiation of the carbonic acid gas, both of these two filters allow this line spectrum to pass therethrough and transmission energy is substantially equal. However, optical energy of the light passing through the broad band filter is divided by a greater bandwidth to calculate the mean intensity, whereas optical energy of the light passing through the narrow band filter is divided by a smaller bandwidth. Consequently, a difference develops between these two mean intensities.
Therefore, the flame can be detected by judging whether or not a difference between the two mean intensities exceeds a threshold value.
However, size of a flame which can be detected by the flame detecting device is inverse proportional to square of a distance between the flame and the flame detecting device. Therefore, wide dynamic range is needed in order to detect a flame in a wide range, that is, from a flame which is located near the device to a flame which is located far away from the device. Regarding flame detection signal and noise, the same consideration is needed. The above threshold value is set in accordance with a distance between the flame detecting device and a main (target) position to be detected. However, when the threshold value is set in accordance with a position to be detected which is near the device, a level of the flame detecting signal for a flame located far way from the device becomes too small and therefore detection of the flame becomes impossible or detection error rises. In contrast, when the threshold value is set in accordance with a position to be detected which is far away from the device, a level of the noise for a flame located near the device becomes too large and therefore detection error rises. Accordingly, there is a problem that it is difficult to detect a flame precisely in a wide range.
Further, when light having a large amount of energy in the infrared ray region, such as solar rays or artificial rays of light such as halogen lamp, is incident, due to that temperature of a filter in the flame detecting device rises, secondary radiation is generated from the filter, and therefore, detection error rises due to the secondary radiation being noise. Further, when stray light is incident in the flame detecting device, the stray light is incident from a side surface of the filter therefore detection error rises.
To solve the problems described above, an object of the present invention is to provide a flame sensor that can accurately detect a flame in a wide range.
Also, another object of the present invention is to provide a flame sensor that can detect a flame without error even when stray light or light having large energy in a range of an infrared region such as solar rays or rays from halogen lamp is incident.
A first aspect of the present invention for accomplishing the objects described above is a flame sensor comprising: a narrow band filter for transmitting only light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame; a broad band filter for transmitting light of a band which includes the band corresponding to the line spectrum and which is broader than the band corresponding to the line spectrum; a first light reception element for converting light transmitted through the narrow band filter to a first electric signal; a second light reception element for converting light transmitted through the broad band filter to a second electric signal; and a judging section for determining whether or not a difference obtained by subtracting a value corresponding to an intensity of the second electric signal from a value corresponding to an intensity of the first electric signal is equal to or greater than a predetermined value, and a ratio between the value corresponding to the intensity of the second electric signal and the value corresponding to the intensity of the first electric signal is within a predetermined range, with the value corresponding to the intensity of the second electric signal and the value corresponding to the intensity of the first electric signal being obtained by assuming that the transmission band width of the broad band filter and the transmission band width of the narrow band filter are equal to each other.
The value corresponding to the intensity of the electric signal may be the intensity of the electric signal itself. Also, the value corresponding to the intensity of the electric signal, that is, the value corresponding to the intensity of the second electric signal and the value corresponding to the intensity of the first electric signal may be a moving average value of intensities of electric signals at a range shorter than a predetermined range which is referenced (based) by a moving average value of intensities of electric signals at the predetermined range.
The electric signal converted by the light reception element can be amplified, or it is also possible that the electric signal converted by the light reception element is not amplified. Further, a flicker component of the flame in a range between 1 Hz and 10 Hz superimposed in the electric signal when light of flame is detected is extracted by filtering processing or the like, and the extracted flicker component is used with being amplified or not amplified. Namely, it is possible that the moving average value is calculated on the basis of the flicker component of light of the flame included in electric signal.
When the spectrum of the light passing through the filter is the continuous spectrum, energy of the rays of light passing (transmitted) through the two filters, namely, the energy of the rays of light passing through the broad band filter and the energy of the rays of light passing through the narrow band filter, is substantially proportional to the transmission bandwidth. Therefore, when assuming that a transmission band width of the broad band filter and a transmission band width of the narrow band filter are equal to each other, that is, when the value corresponding to the intensity of the electric signal is converted to a value obtained by assuming that the transmission band widths of the filters are equal to each other, the difference obtained by subtracting the value corresponding to the intensity of the second electric signal from the value corresponding to the intensity of the first electric signal is less than the predetermined value. Causes of the difference being less than the predetermined value include the shape of the intensity distribution of the spectrum of rays of light passing through the filter and the distance between the band centers of the two filters.
In contrast, when only rays of light of a flame are present, the spectrum passing through the broad band filter and the narrow band filter is mainly only the spectral line because the spectrum of the flame is the line spectrum, and energies passing through the broad band filter and the narrow band filter are substantially equal to each other. Therefore, when assuming that the transmission band width of the broad band filter and the transmission band width of the narrow band filter are equal to each other, the value corresponding to the intensity of the first electric signal is greater than the value corresponding to the intensity of the second electric signal.
Accordingly, it is possible to detect a flame by judging whether or not the difference obtained by subtracting the value corresponding to the intensity of the second electric signal from the value corresponding to the intensity of the first electric signal is equal to or greater than the predetermined value, with the value corresponding to the intensity of the second electric signal and the value corresponding to the intensity of the first electric signal being obtained by assuming that a transmission band width of the broad band filter and a transmission band width of the narrow band filter are equal to each other.
When a flame is detected by using only this difference, as described above, detection of a flame becomes impossible or detection error rises if the predetermined value is not set appropriately.
Next, a ratio of values corresponding to intensities of electric signals is considered. When the spectrum of the light passing through the filter is the continuous spectrum, the energy of the rays of light passing through the broad band filter and the energy of the rays of light passing through the narrow band filter is substantially proportional to the transmission bandwidth thereof. Therefore, a ratio of the value corresponding to the intensity of the second electric signal with respect to the value corresponding to the intensity of the first electric signal, or a ratio of the value corresponding to the intensity of the first electric signal with respect to the value corresponding to the intensity of the second electric signal is equal to a ratio of the transmission band widths of the filters.
In contrast, when only the rays of light of the flame are present, as described above, the energy passing through the broad band filter and the energy passing through the narrow band filter are substantially the same. Accordingly, a ratio between the value corresponding to the intensity of the first electric signal and the value corresponding to the intensity of the second electric signal becomes substantially 1. Therefore, a flame can be detected by judging whether or not the ratio between the value corresponding to the intensity of the second electric signal and the value corresponding to the intensity of the first electric signal is within a predetermined range, for example, is between 1 and the ratio of the transmission band widths.
When a flame is detected by using this ratio, precision of the detection becomes worse, calculation for the detection becomes very difficult, or detection error rises if a value corresponding to denominator of the ratio is a extremely small when calculating the ratio.
However, in the first aspect of the present invention, because judging whether or not both conditions of the difference and the ratio described above are satisfied, detection error does not rise, and it is possible to accurately judge whether or not a flame.
A second aspect of the present invention is a flame sensor comprising: a first blocking filter for blocking light in an infrared ray region; a second blocking filter which is disposed at a light transmitted side of the first blocking filter so as to block stray light; a narrow band filter which is disposed at a light transmitted side of the second blocking filter and which transmits only light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame; a broad band filter which is disposed at the light transmitted side of the second blocking filter and which transmits light of a band which includes the band corresponding to the line spectrum, and which is broader than the band corresponding to the line spectrum; a first light reception element for converting light transmitted through the narrow band filter to a first electric signal; and a second light reception element for converting light transmitted through the broad band filter to a second electric signal.
In the second aspect of the present invention, because the first blocking filter which blocks light in an infrared ray region and the second blocking filter which is disposed at a light transmitted side of the first blocking filter so as to block stray light are used, even in a case of that the flame sensor is disposed at a place where solar rays or artificial rays of light such as halogen lamp are present, the flame can be sensed without being affected by the solar rays or the artificial rays of light. Also, detection error due to stray light does not rise.
The first blocking filter which blocks light in an infrared ray region and the second blocking filter which is disposed at a light transmitted side of the first blocking filter so as to block stray light in the second aspect of the present invention can be used in the first aspect of the present invention. As the light reception elements of the first aspect and the second aspect of the invention, lead selenide, a thermopile, or pyroelectric-type light reception element can be used.
A third aspect of the present invention is a flame sensor comprising: a first filter having a predetermined band for transmitting light, a blocking band for blocking only light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame being formed within the predetermined band; a second filter, having a band width substantially the same as that of the predetermined band, for transmitting light of a band including the band corresponding to the line spectrum; a first light reception element for converting light transmitted through the first filter to a first electric signal; a second light reception element for converting light transmitted through the second filter to a second electric signal; and a judging section for determining whether or not a difference obtained by subtracting a value corresponding to an intensity of the second electric signal from a difference which is obtained by subtracting a value corresponding to an intensity of the first electric signal from the value corresponding to the intensity of the second electric signal is equal to or greater than a predetermined value, and a ratio between the value corresponding to the intensity of the first electric signal and the difference is within a predetermined range, with the value corresponding to the intensity of the second electric signal and the value corresponding to the intensity of the first electric signal being obtained by assuming that a blocking band width of the first filter and a transmission band width of the second filter are equal to each other.
A fourth aspect of the present invention is a flame sensor comprising: a first blocking filter for blocking light in an infrared ray region; a second blocking filter which is disposed at a light transmitted side of the first blocking filter so as to block stray light; a first filter having a predetermined band for transmitting light, a blocking band for blocking only light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame being formed within the predetermined band; a second filter having a band width substantially the same as that of the predetermined band, for transmitting light of a band including the band corresponding to the line spectrum; a first light reception element for converting light transmitted through the first band filter to a first electric signal; and a second light reception element for converting light transmitted through the second band filter to a second electric signal.
In the third and the fourth aspects of the present invention, the difference obtained by subtracting the value corresponding to the intensity of the first electric signal from the value corresponding to the intensity of the second electric signal corresponds to the value corresponding to the intensity of the first electric signal, which is obtained from the light transmitting through the narrow band filter which passes only light of the band corresponding to the line spectrum of carbonic acid gas resonance radiation generated by a flame, of the first and the second aspect of the invention. Therefore, by judging whether or not the difference obtained by subtracting the value corresponding to the intensity of the second electric signal from the difference which is obtained by subtracting the value corresponding to the intensity of the first electric signal from the value corresponding to the intensity of the second electric signal is equal to or greater than the predetermined value, and the ratio between the value corresponding to the intensity of the first electric signal and the difference is within the predetermined range, with the value corresponding to the intensity of the second electric signal and the value corresponding to the intensity of the first electric signal being obtained by assuming that the blocking band width of the first filter and the transmission band width of the second filter are equal to each other, the flame can be detected (determined).
As described above, in the first and the third aspects of the present invention, a flame can be detected precisely in a wide range.
In the second and the fourth aspects of the present invention, a flame can be detected precisely even in a case of that the flame sensor is disposed at a place where light having a large amount of energy in the infrared ray region, such as solar rays or artificial rays of light such as halogen lamp, is incident, or stray light is incident.